JP7832405B2 - Sensor module - Google Patents

Description

This invention relates to a sensor module.

A strain gauge is known that is attached to an object to be measured to detect its strain. The strain gauge includes a resistor for detecting strain, and the material used for the resistor is, for example, a material containing Cr (chromium) or Ni (nickel). Furthermore, the resistor is formed on a substrate made of, for example, an insulating resin, and covered with a protective film (see, for example, Patent Document 1).

This strain gauge can be used as a sensor module, for example, by bonding the base material to a strain-generating body via an adhesive layer.

Japanese Patent Publication No. 2016-74934

However, the strain gauge described above requires a protective film to shield the resistor from moisture and other elements. Therefore, in addition to the process of forming the resistor film on the substrate, a process of covering the resistor with a protective film is necessary. Consequently, using this strain gauge in a sensor module results in an increased overall cost for the sensor module.

This invention has been made in view of the above points, and aims to suppress the increase in the cost of sensor modules.

This sensor module comprises a strain gauge having a flexible resin substrate, a functional layer formed directly on one side of the substrate from a metal, alloy, or metal compound, and a resistor mainly composed of α-Cr formed directly on one side of the functional layer from a film containing Cr, CrN, and _Cr₂N , and a strain-transmitting body that transmits strain to the strain gauge, wherein the functional layer has the function of promoting the crystal growth of α-Cr and forming a film mainly composed of α-Cr, the thickness of the resistor is 0.05 μm to 2 μm, the thickness of the functional layer is 1 nm to 100 nm, and the strain gauge is bonded to the strain-transmitting body with the resistor facing the strain-transmitting body.

According to the disclosed technology, it is possible to suppress the increase in the cost of sensor modules.

This is a plan view illustrating a sensor module according to the first embodiment. This is a cross-sectional view illustrating a sensor module according to the first embodiment.

The following describes embodiments for carrying out the invention with reference to the drawings. In each drawing, identical components are denoted by the same reference numerals, and redundant explanations may be omitted.

<First Embodiment>
Figure 1 is a plan view illustrating a sensor module according to the first embodiment. Figure 2 is a cross-sectional view illustrating a sensor module according to the first embodiment, showing a cross-section along the line A-A in Figure 1. Referring to Figures 1 and 2, the sensor module 5 includes a strain gauge 1, a strain generating body 110, and an adhesive layer 120. The strain gauge 1 includes a base material 10, a resistor 30, and a terminal portion 41.

In this embodiment, for convenience, the sensor module 5 is configured such that the base material 10 side is the upper side or one side, and the strain-generating body 110 side is the lower side or the other side. Furthermore, the surface of each part facing the base material 10 is defined as one surface or the upper surface, and the surface facing the strain-generating body 110 is defined as the other surface or the lower surface. However, the sensor module 5 can be used upside down or positioned at any angle. Also, "planar view" refers to viewing the object from the direction normal to the upper surface 10a of the base material 10, and "planar shape" refers to the shape of the object viewed from the direction normal to the upper surface 10a of the base material 10.

In the strain gauge 1, the base material 10 is a flexible component that serves as the base layer for forming the resistor 30, etc. The base material 10 has an upper surface 10a and a lower surface 10b. The thickness of the base material 10 is not particularly limited and can be appropriately selected depending on the purpose, but for example, it can be approximately 5 μm to 500 μm. In particular, a thickness of 5 μm to 200 μm for the base material 10 is preferable in terms of strain transmission from the surface of the strain-generating body 110, which is joined to the lower surface 10b of the base material 10 via the adhesive layer 120, and dimensional stability against the environment. A thickness of 10 μm or more is even more preferable in terms of insulation.

The base material 10 can be formed from an insulating resin film such as PI (polyimide) resin, epoxy resin, PEEK (polyetheretherketone) resin, PEN (polyethylene naphthalate) resin, PET (polyethylene terephthalate) resin, PPS (polyphenylene sulfide) resin, or polyolefin resin. Note that "film" refers to a flexible material with a thickness of approximately 500 μm or less.

Here, "formed from an insulating resin film" does not prevent the base material 10 from containing fillers or impurities in the insulating resin film. For example, the base material 10 may be formed from an insulating resin film containing fillers such as silica or alumina.

The resistor 30 is a thin film formed in a predetermined pattern on the lower surface 10b of the substrate 10, and is a sensitive part that undergoes a change in resistance when strained. The resistor 30 may be formed directly on the lower surface 10b of the substrate 10, or it may be formed on the lower surface 10b of the substrate 10 via another layer. For convenience, in Figure 1, the resistor 30 is shown with a textured surface.

The resistor 30 can be formed from, for example, a material containing Cr (chromium), a material containing Ni (nickel), or a material containing both Cr and Ni. That is, the resistor 30 can be formed from a material containing at least one of Cr and Ni. An example of a Cr-containing material is a Cr multiphase film. An example of a Ni-containing material is Ni-Cu (nickel copper). An example of a material containing both Cr and Ni is Ni-Cr (nickel chromium).

Here, a Cr multiphase film is a film in which Cr, CrN, _Cr₂N , etc., are mixed. The Cr multiphase film may contain unavoidable impurities such as chromium oxide.

The thickness of the resistor 30 is not particularly limited and can be appropriately selected depending on the purpose, but for example, it can be about 0.05 μm to 2 μm. In particular, a thickness of 0.1 μm or more is preferable because it improves the crystallinity of the crystals constituting the resistor 30 (for example, the crystallinity of α-Cr), and a thickness of 1 μm or less is even preferable because it reduces cracks in the film and warping from the substrate 10 caused by internal stress in the film constituting the resistor 30.

For example, if the resistor 30 is a Cr multiphase film, the stability of the gauge characteristics can be improved by using α-Cr (alpha-chromium), a stable crystalline phase, as the main component. Furthermore, by using α-Cr as the main component of the resistor 30, the gauge factor of the strain gauge 1 can be set to 10 or higher, and the temperature coefficient of gauge factor TCS and the temperature coefficient of resistance TCR can be set within the range of -1000 ppm/°C to +1000 ppm/°C. Here, "main component" means that the substance in question accounts for 50% or more by mass of the total substances constituting the resistor. From the viewpoint of improving gauge characteristics, it is preferable that the resistor 30 contains 80% by weight or more of α-Cr. Note that α-Cr is Cr with a bcc structure (body-centered cubic lattice structure).

The terminal portion 41 extends from both ends of the resistor 30 and, in a plan view, is wider than the resistor 30 and formed in a roughly rectangular shape. The terminal portion 41 is a pair of electrodes for outputting the change in the resistance value of the resistor 30 caused by strain to the outside.

For example, a portion of the side surface of each terminal portion 41 is exposed to the outside of the strain gauge 1, and an external connection lead wire or the like is joined to the exposed portion. However, as long as a portion of each terminal portion 41 is exposed to the outside of the strain gauge 1, the exposed portion does not have to be the side surface of each terminal portion 41. For example, through holes or notches may be provided in the base material 10 to expose a portion or all of the upper surface of each terminal portion 41.

The resistor 30 extends from one terminal portion 41 in a zigzag pattern and connects to the other terminal portion 41. The upper and side surfaces of the terminal portions 41 may be covered with a metal that has better solderability than the terminal portions 41. Although the resistor 30 and the terminal portions 41 are given different reference numerals for convenience, they can be integrally formed from the same material in the same process.

In the sensor module 5, the lower surface 10b of the base material 10 is bonded to the upper surface 110a of the strain generating body 110 via the adhesive layer 120. That is, the strain gauge 1 is bonded to the strain generating body 110 with the resistor 30 facing the strain generating body 110.

The strain-generating body 110 is formed from a metal such as Fe, SUS (stainless steel), or Al, or a resin such as PEEK. It deforms in response to the applied force and transmits the resulting strain to the strain gauge 1. The strain gauge 1 can detect the strain generated in the strain-generating body 110 as a change in the resistance value of the resistor 30.

The adhesive layer 120 is sandwiched between the strain gauge 1 and the strain-generating body 110, and covers the resistor 30. The adhesive layer 120 is not particularly limited as long as it is a material that has the function of bonding the strain gauge 1 and the strain-generating body 110, and can be appropriately selected according to the purpose. For example, epoxy resin, modified epoxy resin, silicone resin, modified silicone resin, urethane resin, modified urethane resin, etc., can be used. Alternatively, materials such as bonding sheets may be used. However, if the strain-generating body 110 is a conductor, it is necessary to select an insulating material for the adhesive layer 120. The thickness of the adhesive layer 120 is not particularly limited and can be appropriately selected according to the purpose, but for example, it can be approximately 0.1 μm to 50 μm.

To manufacture the strain gauge 1, first, a base material 10 is prepared, and a resistor 30 and terminal portion 41 in the planar shape shown in Figure 1 are formed on the lower surface 10b of the base material 10. The material and thickness of the resistor 30 and terminal portion 41 are as described above. The resistor 30 and terminal portion 41 can be integrally formed from the same material.

The resistor 30 and terminal portion 41 can be formed, for example, by depositing a film using a magnetron sputtering method targeting a raw material capable of forming the resistor 30 and terminal portion 41, and then patterning it using photolithography. Alternatively, the resistor 30 and terminal portion 41 may be deposited using reactive sputtering, vapor deposition, arc ion plating, pulsed laser deposition, or other methods instead of magnetron sputtering.

From the viewpoint of stabilizing the gauge characteristics, it is preferable to vacuum-deposit a functional layer with a thickness of approximately 1 nm to 100 nm onto the lower surface 10b of the substrate 10 as an underlayer, for example, by conventional sputtering, before depositing the resistor 30 and terminal portion 41. The functional layer is then patterned with the resistor 30 and terminal portion 41 into the planar shape shown in Figure 1 by photolithography after the resistor 30 and terminal portion 41 have been formed on the entire lower surface of the functional layer.

In this application, the functional layer refers to a layer having the function of promoting crystal growth of the resistor 30, which is at least the upper layer. Preferably, the functional layer further has the function of preventing oxidation of the resistor 30 by oxygen and moisture contained in the substrate 10, and the function of improving the adhesion between the substrate 10 and the resistor 30. The functional layer may further have other functions.

Since the insulating resin film constituting the base material 10 contains oxygen and moisture, and especially when the resistor 30 contains Cr, the Cr forms a self-oxidizing film, it is effective for the functional layer to have a function that prevents oxidation of the resistor 30.

The material of the functional layer is not particularly limited as long as it has the function of promoting crystal growth of at least the upper layer, the resistor 30. It can be appropriately selected according to the purpose. Examples include one or more metals selected from the group consisting of Cr (chromium), Ti (titanium), V (vanadium), Nb (niobium), Ta (tantalum), Ni (nickel), Y (yttrium), Zr (zirconium), Hf (hafnium), Si (silicon), C (carbon), Zn (zinc), Cu (copper), Bi (bismuth), Fe (iron), Mo (molybdenum), W (tungsten), Ru (ruthenium), Rh (rhodium), Re (rhenium), Os (osmium), Ir (iridium), Pt (platinum), Pd (palladium), Ag (silver), Au (gold), Co (cobalt), Mn (manganese), and Al (aluminum), an alloy of any of these metals, or a compound of any of these metals.

Examples of _the alloys mentioned above include FeCr, TiAl, FeNi, NiCr, and CrCu. Examples of the compounds mentioned above include TiN, TaN, _Si3N4 , _TiO2 , _Ta2O5 , _{and SiO2} .

The functional layer can be deposited using a conventional sputtering method, for example, by targeting a raw material capable of forming a functional layer and introducing argon (Ar) gas into a chamber. By using the conventional sputtering method, the functional layer is deposited while etching the lower surface 10b of the substrate 10 with Ar, thus minimizing the amount of functional layer deposited and achieving improved adhesion.

However, this is merely one example of a method for forming a functional layer, and the functional layer may be formed by other methods. For example, a method may be used in which the lower surface 10b of the substrate 10 is activated by plasma treatment using Ar or the like before forming the functional layer to improve adhesion, and then the functional layer is formed using a vacuum deposition method with a magnetron sputter.

The combination of materials for the functional layer and the resistor 30 and terminal portion 41 is not particularly limited and can be appropriately selected according to the purpose. For example, it is possible to use Ti as the functional layer and deposit a Cr multiphase film mainly composed of α-Cr (alpha-chromium) as the resistor 30 and terminal portion 41.

In this case, for example, the resistor 30 and terminal portion 41 can be formed by magnetron sputtering using a raw material capable of forming a Cr multiphase film as the target, with Ar gas introduced into the chamber. Alternatively, the resistor 30 and terminal portion 41 may be formed by reactive sputtering using pure Cr as the target, with an appropriate amount of nitrogen gas introduced into the chamber along with Ar gas.

In these methods, a functional layer made of Ti (Ti) initiates the growth surface of the Cr multiphase film, enabling the formation of a Cr multiphase film primarily composed of α-Cr, which has a stable crystalline structure. Furthermore, the diffusion of Ti from the functional layer into the Cr multiphase film improves the gauge characteristics. For example, the gauge factor of strain gauge 1 can be set to 10 or higher, and the gauge factor temperature coefficient TCS and resistance temperature coefficient TCR can be set within the range of -1000 ppm/°C to +1000 ppm/°C. Note that when the functional layer is formed from Ti, the Cr multiphase film may contain Ti or TiN (titanium nitride).

Furthermore, when the resistor 30 is a Cr multiphase film, the functional layer made of Ti has all of the following functions: promoting crystal growth of the resistor 30, preventing oxidation of the resistor 30 by oxygen and moisture contained in the substrate 10, and improving the adhesion between the substrate 10 and the resistor 30. The same applies when Ta, Si, Al, or Fe is used instead of Ti as the functional layer.

Thus, by providing a functional layer beneath the resistor 30, it becomes possible to promote crystal growth in the resistor 30, enabling the fabrication of a resistor 30 with a stable crystalline phase. As a result, the stability of the gauge characteristics in the strain gauge 1 can be improved. Furthermore, the diffusion of the material constituting the functional layer into the resistor 30 further improves the gauge characteristics of the strain gauge 1.

To manufacture the sensor module 5, after fabricating the strain gauge 1, one of the above materials, which will serve as the adhesive layer 120, is applied to the lower surface 10b of the base material 10 and/or the upper surface 110a of the strain generating body 110. Then, the lower surface 10b of the base material 10 is placed opposite the upper surface 110a of the strain generating body 110, and the strain gauge 1 is positioned on the strain generating body 110 with the applied material in between. Alternatively, a bonding sheet may be sandwiched between the strain generating body 110 and the base material 10.

Next, the strain gauge 1 is pressed against the strain generating body 110 and heated to a predetermined temperature, curing the applied material and forming an adhesive layer 120. This bonds the upper surface 110a of the strain generating body 110 to the lower surface 10b of the base material 10 via the adhesive layer 120, completing the sensor module 5. The sensor module 5 can be applied to measuring, for example, load, pressure, torque, acceleration, etc.

In this way, in the sensor module 5, the strain gauge 1 is bonded to the strain generating body 110 with the resistor 30 facing the strain generating body 110. In other words, the upper surface of the resistor 30 is covered by the base material 10. By covering the upper surface of the resistor 30 with the base material 10, mechanical damage to the resistor 30 can be prevented. Furthermore, by covering the upper surface of the resistor 30 with the base material 10, the resistor 30 can be protected from moisture and other elements.

Incidentally, as mentioned above, conventional sensor modules have a structure in which a strain gauge is fabricated by forming a resistor and a protective film on a substrate, and the substrate side of the strain gauge is bonded to the strain-generating body via an adhesive layer. In this case, in addition to the process of forming the resistor film on the substrate, a process of covering the resistor with a protective film is required, which leads to the problem of increased cost for sensor modules using strain gauges.

In contrast, in the sensor module 5, the strain gauge 1 has the resistor 30 facing the strain generating body 110 and bonded to the strain generating body 110, and the upper surface of the resistor 30 is covered with the base material 10. That is, in the sensor module 5, the base material 10 also serves the role of the protective film in conventional sensor modules, eliminating the need for the process of covering the resistor with a protective film, thus shortening the manufacturing time. This helps to suppress the cost increase of the sensor module 5.

Although preferred embodiments have been described in detail above, the invention is not limited to the embodiments described above, and various modifications and substitutions can be made to the embodiments described above without departing from the scope of the claims.

1 Strain gauge, 5 Sensor module, 10 Substrate, 10a Top surface, 10b Bottom surface, 30 Resistor, 41 Terminal section, 110 Strain generating element, 110a Top surface, 120 Adhesive layer

Claims (5)

A strain gauge comprising a flexible resin substrate, a functional layer formed directly on one surface of the substrate from a metal, alloy, or metal compound, and a resistor mainly composed of α-Cr formed directly on one surface of the functional layer from a film containing Cr, CrN, and _Cr₂N ,
The strain gauge comprises a strain-generating body that transmits strain to the aforementioned strain gauge,
The functional layer has the function of promoting the crystal growth of α-Cr and forming a film mainly composed of α-Cr.
The thickness of the resistor is 0.05 μm or more and 2 μm or less.
The thickness of the functional layer is 1 nm or more and 100 nm or less.
The strain gauge is a sensor module in which the resistor is attached to the strain generating body with the resistor facing the strain generating body. The strain gauge comprises a pair of electrodes electrically connected to the resistor,
The sensor module according to claim 1, wherein part or all of the faces of the pair of electrodes opposite to the strain-generating body are exposed within an opening provided in the substrate. The device has an adhesive layer sandwiched between the strain gauge and the strain generating body,
The sensor module according to claim 1 or 2, wherein the adhesive layer covers the resistor. The sensor module according to claim 3, wherein the adhesive layer is formed from epoxy resin, modified epoxy resin, silicone resin, modified silicone resin, urethane resin, or modified urethane resin. The sensor module according to claim 3 or 4, wherein the thickness of the adhesive layer is 0.1 μm or more and 50 μm or less.